Those of skill in the art will understand acoustic immittance to be defined broadly herein as referring to either or both of acoustic impedance or its logical and numerical inverse, acoustic admittance. These two acoustic characteristics are characterized as being numerical inverses of one another in that an acoustic impedance measurement of 0.2 corresponds to an acoustic admittance measurement of 5.0. In other words, admittance=1/impedance and impedance=1/admittance. The invention is not so limited, however, to such a precise logical or mathematical construct, and the appended claims are intended broadly to cover acoustic impedance, admittance, and/or immittance, however measured or otherwise characterized.
The measurement of aural acoustic immittance has been performed for many years, and it is a routine that is common in the initial test battery run on patients who visit a physician with a complaint of ear problems. Generally this physician is one with a specialty in Ear Nose and Throat pathology. The test is commonly administered by an audiologist. Audiologists are people who are trained in the function, pathology and testing of ears. The specific types of equipment used to administer this test are commonly called ‘impedance testers’, ‘admittance testers’, or ‘middle ear analyzers’.
Impedance testers commonly do a set of tests such as tympanometry, acoustic reflex test, acoustic reflex decay and Eustachian tube function tests. Primarily these tests are run to test the condition of the bones and ligaments between the tympanic membrane (eardrum) and the inner ear. The purpose of this ‘middle ear’ is to conduct energy received by the tympanic membrane through the air-filled middle ear to the fluid-filled inner ear, where it is separated into discrete frequency components and transduced into neural energy for conduction to the brainstem.
Test instruments used to make measurements on the middle ear typically measure the manner that it conducts the energy from the tympanic membrane to the inner ear. They measure the acoustic admittance using a probe with a plastic tip inserted into the ear canal. The plastic tip forms an air-tight seal in the ear canal. The air pressure typically is varied linearly (although alternative sweeps, e.g. a non-linear sweep that shortens test time, are sometimes used) in the ear canal from slight pressure to slight vacuum, and an acoustic test signal emitted from the probe will be measured using a microphone in the probe during this varying of the air pressure. When an electronic circuit is used to ‘read’ the microphone and then to vary the probe tone (the emitted pure tone) in order to keep the sound pressure level in the ear canal constant, the error correction signal in this feedback loop will be proportional to the acoustic admittance of the middle ear system. Both the magnitude and phase of this admittance can be measured, although it is common to measure and report only the magnitude of the measurement. Tympanometry is useful mainly to determine if there are problems with middle ear movement. See, for example, Liden, G., The scope and application of current audiometric tests, Journal of Laryngology and Otology, Volume 83, pp 507-520 (1969). Also see, for example, Jerger, J., Clinical experience with impedance audiometry. Archives of Otolaryngology, Volume 92, pp 311-324 (1970).
Originally these test instruments were stand-alone instruments, wholly contained stimulus and measurement instruments in one box that encompassed all necessary elements to perform the tests. These instruments had a power supply and derived their operating power directly from the power mains. In recent years these instruments have been connected to personal computers. A suitably programmed, special-purpose computer executing software instructions residing in memory, for example, can provide the user interface including a tympanographic display, provide for long-term storage of data, and even interface with other data stores such as patient management and billing systems. Although instruments have been computer-connected, they have kept their own power supplies and source energy directly from the mains supply.
Tympanometry is a measure of the components of stiffness of the middle ear and thus it evaluates middle ear function. This test can be helpful in detecting fluid in the middle ear, negative middle ear pressure, disruption of the ossicles, tympanic membrane perforation, and otosclerosis.
Tympanometry is also used for the evaluation of eustachian tube function. It is usually done by the same instrument that performs acoustic reflex testing.
Acoustic reflex testing consists of subjecting the ear to a loud pure tone or noise sound and determining whether and to what extent the sound causes the stapedius muscle to tighten the stapes. This in turn will stiffen the middle ear system, causing a measureable change in the impedance of that system. Acoustic reflexes are mainly useful as a non-subjective method of evaluating certain pathways from the ear to the brainstem and back, as the stapes should tighten for a sufficient level of sound. The absence of acoustic reflexes can be a sign of middle ear problems or of brainstem dysfunction.
To perform the tympanometry test, a probe with a soft plastic tip is placed into the ear canal. The plastic tip will seal the ear canal enabling the air pressure at the tympanic membrane to be varied. The air pressure in the sealed ear canal is varied in a systematic way from a small pressure to a small vacuum, or from vacuum to pressure, in a cyclic or loop technique that involves plural serial triple steps including measurements, computations, and adjustments, the cycle or loop being repeated until the measurement results finally converge. Conventional instruments use a microphone in the probe to measure the sound pressure level of a pure tone signal emitted by the probe and correct this sound pressure level so that it is held to a constant, unvarying value while the air pressure is changed. The error signal in this closed-loop correction system is directly proportional to the acoustic admittance of the middle ear system.
The result of the test conventionally is recorded in a visual output, called a tympanogram. If there is fluid in the middle ear, the middle ear system will not properly move freely, and the line on the tympanogram will be relatively flat. If there is air in the middle ear (the normal condition) but the air is at a higher or lower pressure than the surrounding atmosphere, the line on the tympanogram will be shifted to the left or right in position.